In the completion of wells drilled in the course of the exploration for and production of oil and natural gas, the borehole is typically cased and cemented. This generally occurs in several stages. An initial section of the well is drilled with a large diameter bit and into this section large diameter casing is set. This initial length of casing is termed "surface casing". The annulus intermediate the borehole and the surface casing is then filled with cement. Following this, a smaller diameter drill string is passed through the initial section of casing and an additional section of the well is drilled and then cased and cemented. This process continues, often through three or more stages, until the desired depth has been attained.
A prime reason for utilizing cemented casing in the completion of a well is to isolate from fluid communication with one another the various strata or horizons through which the borehole passes. Absent this step a number of undesirable situations could arise due to this fluid communication; among these are that valuable hydrocarbons could be lost from a high pressure reservoir stratum to a lower pressure "thief" stratum and it could be difficult to direct formation treatments to a selected stratum. The cemented casing also serves to provide structural support to the wellbore to prevent the collapse of any portion of the formation into the wellbore.
After having set the casing, it is necessary to provide for fluid communication between the wellhead and one or more of the subterranean strata. In most instances this is accomplished by a technique known as perforation, in which a series of holes are formed through the casing and cement into the desired stratum. Through these perforations oil and gas can pass from the reservoir to production tubing. Also, fluids can be injected into the stratum through the perforations in the process of stimulating the production of oil and gas.
One of the most common and long standing problems associated with the completion of oil and gas wells is the occurrence of fluid communication between various otherwise isolated strata. This is generally caused by leakage along the cemented annulus due to the cement not forming a perfect seal between the wellbore and the casing. This can cause the loss of hydrocarbons from the reservoir, decrease the effectiveness of stimulation treatments such as formation fracturing and acidizing, and prevent injected drive fluids such as water and carbon dioxide from displacing oil and gas in an efficient manner. In some such situations, the existence of this fluid communication between otherwise isolated strata in a well can result in the loss of very significant sums of money.
The cause of fluid communication between strata where none existed prior to the drilling of the well is most often the result of an imperfect cement job. Less often, however, this problem is the result of fracturing of the formation in the course of drilling or treating the well.
In the cementing process, cement is pumped through the casing to the bottom of the cased portion of the borehole, where it passes into the annulus between the casing and the wellbore. As pumping is continued, it is desired that cement should fill the annulus and cause the drilling mud to be displaced upward and out of the annulus. However, in this process some of the mud may not be displaced, leaving passageways or channels in the cemented annulus through which fluids can readily flow. This is called "channeling". It is also believed that a too-rapid loss of pressure in the cemented annulus during curing of the cement can result in reduced resistance of the cement to fluid flow. Even though the utmost care may be taken, the resulting cement job can still be imperfect on occasion. Remedial cementing, often termed "cement squeezing", is utilized to introduce cement into channeled regions of the annulus following the initial cementation of the annulus. Such cement repair operations are expensive and are not always successful.
One of the most troublesome aspects of fluid communication along the annulus is the difficulty that has been experienced heretofore in detecting it. Most traditional methods of making measurements in wells rely on logging. In well logging, a condition monitoring assembly, termed a "sonde", is lowered into a case or open wellbore at the end of a wireline and measurements of one or more features of the well and its surroundings are made as a function of depth. A major drawback in the use of cased hole logging for monitoring a condition external to the casing, such as fluid flow within the annulus, is that the existence and magnitude of such conditions can in most instances only be inferred, and not measured directly. For example, common techniques for monitoring flow along the annulus include the taking of temperature and noise logs. From data so obtained the existence of fluid passage through the annulus can in many instances be discovered from anomolous temperature gradients and noise shown by the log. However, such techniques are expensive, often don't provide definitive data and generally require other operations on the well to cease during the time of logging. Further, these cased hole logging techniques are largely insensitive to fluid flow between strata by a pathway other than through the annulus.
The most common direct technique for monitoring fluid flow along the annulus involves the injection of a radioactive tracer into the horizon of interest through perforations in the casing. A gamma-ray log is then run to detect any passage of fluid from the horizon into which the injection took place to other regions along the wellbore. This method is disadvantageous in that it is rather insensitive to the detection of fluid communication other than along the annulus. Further, this technique requires the use of a radioactive material.
Still another method for determining the existence of fluid flow between strata relies upon pressure measurements. The completion of a well generally requires that the various producing horizons within the formation be perforated and connected to the surface in fluid isolation from the remainder of the producing formations through a tubing and packer system. Thus, each potential hydrocarbon bearing horizon may be produced and treated individually. By monitoring the pressure in the tubing system associated with each perforated horizon, information regarding fluid flow away from or into each horizon can be inferred. This method is disadvantageous in that only those strata in fluid communication with the surface can be monitored. An additional disadvantage is that monitoring formation pressure through a production string generally requires the cessation of production or treatment on the formation for which the pressure measurement is being made.
It would be desirable to provide a simple and accurate method for monitoring a well for fluid communication along the wellbore between strata intersected by the wellbore. It is further desirable that such a system be adapted to provide fluid communication monitoring continuously and in such a manner that production and formation treating operations may be carried on simultaneously with the monitoring. It is also desirable that such a system not be dependent upon the existence of fluid communication between the strata of interest and the interior of the casing.